1. Technical Field
The present invention relates generally to plasma devices and in particular to plasma devices used for etching a substrate or depositing material on a substrate. Still more particularly, the present invention relates to improvements reducing size and cost of a plasma producing device used in conjunction with a substrate while improving the uniformity of the plasma produced at the substrate workpiece.
2. Description of the Related Art
Plasma producing devices are commonly used in plasma assisted processing to etch geometries associated with device formation into a substrate or to deposit layers of material on the substrate. Such devices are disclosed in U.S. Pat. Nos. 5,196,670 and 4,483,737, which are incorporated herein by reference.
One class of plasma producing devices, described in U.S. Pat. No. 5,196,670, employs a magnetic field in conjunction with microwave energy. In these devices, plasma is produced from a working gas as a result of the interaction of a magnetic field with an electric field. A microwave waveguide is used to inject microwaves, which have an associated electric field, into an evacuable chamber containing the working gas. The microwaves propagate into the chamber in a direction substantially perpendicular to the surface of the substrate or workpiece. The electric field associated with the microwaves is perpendicular to the direction of propagation, radially outward from a line following the direction of propagation of the microwaves.
A magnetic field is provided, causing plasma electrons to rotate around the direction of propagation at right angles with the magnetic field. At the plane of resonance, the point at which the electric field and the rotation of plasma electrons are in phase, the microwave electric field constantly accelerates the rotating plasma electrons. The energy of this acceleration dissociates molecules of the working gas into atoms and removes electrons from the atoms, creating ions and additional electrons. The ions then diffuse and impinge upon the surface of the workpiece.
The requisite magnetic field may be provided by a single permanent magnet situated above the outlet of the microwave waveguide into the chamber. An adjusting element may be provided between the magnet and the waveguide to vary the spatial relationship between the magnet and the waveguide opening, thus altering the location of the plane of resonance or "resonance zone" within the chamber.
While use of a permanent magnet situated over the waveguide opening to the chamber has advantages over other plasma producing methods, a permanent magnet of the size required to provide the requisite magnetic field may be very expensive. Furthermore, no ability exists to shape, direct, or otherwise control the magnetic field produced by the permanent magnet, or to minimize stray magnetic fields around the magnet. Control over both the direction of the magnetic field produced by the permanent magnet and stray magnetic fields would be advantageous to improve plasma uniformity at the workpiece. Prior art attempts to obtain plasma uniformity have focused on achieving a uniform magnetic field, which requires very large and bulky magnets. It would be advantageous to minimize the magnetic field near the workpiece, resulting in an inherently uniform plasma.
Another drawback of the prior art use of permanent magnets in plasma producing devices relates to the microwave waveguide. Use of a standard waveguide constrains placement of the permanent magnet with respect to the chamber. As the magnet face is moved further from the chamber, larger, more expensive magnets are required to produce the requisite magnetic field. It would be advantageous, therefore, to be able to reduce the distance between the face of the permanent magnet and the chamber.
Additionally, the prior art teaches placement of the waveguide directly on a window in the evacuable chamber. Microwaves are thus transmitted directly from the waveguide into the chamber through the window, and transition from the rectangular transmission mode to the circular transmission mode inside the chamber. Thus, it would be advantageous to control placement of the waveguide opening relative to the chamber window to facilitate plasma uniformity and tuning.